[clang] [clang][ASTImporter] fix assert fail due to offset overflow (PR #79084)

[Ding Fei via cfe-commits]

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@@ -9896,9 +9903,13 @@ Expected<FileID> ASTImporter::Import(FileID FromID, bool IsBuiltin) {
         // FIXME: The filename may be a virtual name that does probably not
         // point to a valid file and we get no Entry here. In this case try with
         // the memory buffer below.
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danix800 wrote:

Invalid file could exist in real case (causes no Entry) while `SourceLocation` overflow is normally caused by cumulative effect of multiple (large enough) files imported so I think these two are not connected.

https://github.com/llvm/llvm-project/pull/79084